


kaleidoscope

by avosettas



Category: Undertale (Video Game)
Genre: Ecto-Body (Undertale), Ecto-Breasts (Undertale), Ecto-Genitalia (Undertale), Ecto-Vagina (Undertale), Errortale Sans (Undertale), Errortale Sans/Underswap Sans (Undertale), Established Relationship, First Time, Gentle Sex, Haphephobia, M/M, Non-Penetrative Sex, Praise Kink, Underswap Sans (Undertale), Vaginal Fingering
Language: English
Status: Completed
Published: 2020-12-27
Updated: 2020-12-27
Packaged: 2021-03-10 22:00:22
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 2,619
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/28364304
Author URL: https://archiveofourown.org/users/avosettas/pseuds/avosettas
Summary: "kaleidoscope" is derived from the Ancient Greek word καλός (kalos), "beautiful, beauty", εἶδος (eidos), "that which is seen: form, shape" and σκοπέω (skopeō), "to look to, to examine", hence "observation of beautiful forms." (wikipedia)~Error asks Blue to have sex with him. Y'know, for exposure therapy?
Relationships: Sans/Sans (Undertale)
Comments: 8
Kudos: 144





	kaleidoscope

**Author's Note:**

> [vibrates with the force of my love for error] hi i have so many ideas to gently wreck this boy. i will gently wreck this boy.   
> anyway. baby's first smut that she's actually proud of hhhhhh

“So you know h-how we’ve been doing that… e-exposure therapy thing?” At Blue’s nod, Error continues, “I w-want you to fuck me.” 

Blue stares. His mouth opens and closes for a moment, before settling on hanging open, and already his tongue has unconsciously decided to make itself at home in his mouth. Error looks so nervous - standing above Blue in the AntiVoid, he’s glitching much more than usual, although not so much that Blue worries he’ll crash. He’s drawn his head back into his hoodie, like he’s trying to disappear into it. 

“I’m...not opposed,” Blue says after a moment. “But are you sure?” 

“Of course I’m f-fucking sure!” Error snarls, throwing himself onto the beanbag besides Blue. “I’ve b-been really good lately, and th-this is the next step, I think.” 

Blue sighs, and then, testing the waters, leans back into the beanbag, falling into the well created by Error’s weight. Error only startles slightly at the contact of their sides - much, much better than he’d been a year prior, when they’d begun this journey of exposure therapy. 

“Okay,” Blue agrees finally. “How -” 

“I w-want you to be on top,” Error interrupts bluntly. Blue must look as confused as he feels, because despite his blush, Error rolls his eyes. “I-if I crash or something, th-then you w-won’t be left needy, dumbass. I’d b-be a pretty shit boyfriend i-if that happened.” 

“No, you wouldn’t,” Blue says automatically, because as much as he wants to be in control, it’s much more important that Error is comfortable. 

Error’s face flushes… not darker, but the blue hue of his blush takes on a slightly yellow shade, now. Finally, he mumbles, not quite meeting Blue’s eyes, “I’d st-still rather you… be in c-control…” 

“Okay.” Blue pulls his gloves off before anything else, and then his bandana, setting them to the side, and casually continues, “You should pick a safeword. Even if you don’t think you’ll need it, I’d be more comfortable if you gave me one.” 

“Fine,” Error replies petulantly, but Blue can see how tense he is, despite his slumped position on the beanbag. “...I g-guess make it…” He scrunches up his face a bit. It’s so cute, Blue has to look away to keep from commenting. Plenty of time for that later, when Error is frightened by touch and will need Blue’s voice to keep him distracted. 

“We could just use ‘stop’,” Blue shrugs. 

“Fine,” Error repeats. 

Blue looks at him for a moment. He looks no tenser than he usually does when they practice this sort of thing, although his face is a mixture of different colors due to embarrassment. That’s new. He still looks as though he’s trying to disappear into his baggy clothes, but again, it’s not abnormal. 

With that in mind, Blue leans forward, slowly so Error can stop him if needed, and kisses him. 

Kissing Error isn’t new, but the strangeness is something that will always stick out, because Error has five tongues when he allows them to manifest. They’re all smaller than Blue’s tongue and deep blue like Error’s magic. It always feels strange to kiss him; it’s like they have a mind of their own. 

When Blue pulls away, Error seems fine, aside from the haziness of his blown out eye lights. He strokes a thumb down the blue marks on Error’s face, listening to the soft, rattling purr it makes Error let out. He won’t meet Blue’s eyes, but Blue responds with his own rumble as he leans down closer and nuzzles into the crook of Error’s neck. 

“I love you,” Blue mumbles into his shoulder. 

Error only responds with a little groan, and a mumble that sounds vaguely like “too”. Blue chuckles at that, and nuzzles at Error’s neck a bit more. “Can I bite?” 

“...Not h-hard…” 

“Of course not.” Blue has to push Error’s scarf and the neck of his hoodie out of the way a bit, but then he’s met with dull, black bone. It’s not particularly nice; Error doesn’t take the greatest care of himself, and he has many scars from fighting enemies across the multiverse. But Blue loves the sight of it anyway, and he can’t stop himself from pressing a toothy kiss to the bones before opening his jaw and sinking his teeth in. 

Error whines beneath him, but Blue doesn’t remove his teeth yet, instead lapping at the bones for a moment before moving to nuzzle at the mark again. Like Error’s blush, the bruise is a multitude of colors; mostly blue, with hints of yellow and red, but it doesn’t last. Blue is too quick for that, running his tongue over the mark back and forth, calming the marred bone. 

He’s still keeping a close eye on Error, even though he’s sure it will be obvious if the other crashes; right now, he’s simply fizzling a bit, but otherwise seems to be doing fine. 

“I’m so proud of you,” Blue murmurs against his neck, moving his hands to the bottom of Error’s ribs. There’s still a shirt preventing direct bone-on-bone contact, but Error shivers nonetheless. “You’re doing so well.” One hand slips to the hem of the shirt, mostly resting in the empty space between Error’s ribs and pelvis for now. “Let me take this off, hm?” 

Error nods frantically, and Blue smiles at him. He slips both hands under the shirt, pushes it up, but not over Error’s head. His hoodie and scarf still prevent it from coming completely off, but Blue hesitates a bit with that - he wants to see how Error will handle this first. 

Blue’s left hand cradles Error’s sternum, the palm touching his chipped xiphoid process as he lets his thumb ghost over the surface. Error lets out a long sigh, inhaling shakily when Blue rubs at the inside with his index fingers. “T-too much,” he mumbles. Blue immediately removes his hand, but Error grabs it. 

“G-gonna try… that m-magic thing.” 

“Ecto?” Blue replies curiously. He’s not quite following Error’s train of thought here, but maybe he’s a little fuzzed by arousal. 

Error lets go of his hand, twisting on the beanbag to cover his ribs with his hoodie a bit. “I-I’m better when there’s n-no bone-on-bone c-contact, r-remember?” As he speaks, his clothes fill out a bit; Blue watches with barely disguised interest that he’s sure Error can see, considering how much darker his face flushes. 

“Okay,” Blue concedes as Error releases the hoodie, leaving Blue free to pull it away and expose his torso again. “If it’s still too much, tell me, okay?” 

Error rolls his eyes, but there’s some amusement in his voice as he says, “Y-yeah, yeah, I know.” 

Blue peels back the open hoodie again, and presses Error’s shirt up towards his neck, marveling at his manifested body all the while. Like the rest of his magic, it’s a dark shade of blue, and it glitches slightly like the rest of his body. 

“So pretty, Error,” Blue tells him, bringing his hands to Error’s hips and rubbing his thumbs there, firm enough to almost feel the iliac crest below, but definitely not enough to hurt. “How does it feel?” 

“...Different,” Error replies, a little breathless. Blue smiles at that. It’s not a response Error has given him before, during their ‘exposure sessions’, but he seems to be more aroused than anything. He moves his hand from Error’s hip to just below his breast in a smooth sweep, and Error twitches beneath him. 

Maybe Blue’s just a little mean, because he swipes his hand over Error’s belly again, laughing softly when he arches against Blue’s hand and twitches when Blue rubs his thumb against it. “You’re so sensitive,” Blue giggles, running his hand along the soft pudge of Error’s manifested stomach, and Error opens his mouth to reply, probably some defensive retort, before Blue sighs, “I love it. It’s so cute!” 

“I-I’m…” Error doesn’t finish his argument, just turns his head to avoid eye contact. Blue would like him to look at him, to watch Blue make him feel good, but another time. For now, Blue leaves his left hand on Error’s hip, hoping to ground him if he needs it, and moves the other one up to his breast, finally deciding to leave his belly alone.

He doesn’t squeeze yet, just traces his right hand in a large circle a few times. Error twists beneath him, pressing his right side up towards Blue a bit, as if to make Blue pay more attention to him. Blue smiles, instead pushing Error back down with his right hand, settling it where it had originally been, just beneath Error’s breast. 

He finally moves his left hand instead, to properly squeeze Error’s breast, and Error outright moans. “Oh, you like that?” Blue can’t help the amused titter in his voice. 

Error doesn’t respond, only covers his face with hands - a shame, in Blue’s opinion, because he wants to see _everything_. But Error’s comfort is still more important, always will be, even outside of their ‘exposure therapy’ dates. 

So he leaves Error’s hands be, and rubs his thumb in wide circles on Error’s breast instead, skirting his nipple, and watching to see if he can puzzle out any expressions behind his hands. Error’s skull is a mess of colors; blue, with yellow on the edges, turning to red in the center near his nose, and Blue can’t resist leaning up to press a kiss to his nasal ridge, right where there’s an opening between his hands. 

“You’re being so good for me, Error,” Blue tells him, still gently groping his boyfriend with both hands as he speaks, and Error whines behind his hands. “Being so responsive and making all those cute noises…” 

Error twitches beneath him. “St-stop teasing.” 

“That’s half the fun though,” Blue pouts, but with a last squeeze that makes Error jolt, he relents and lets his hands skim Error’s waist instead. “Can I take these off?” 

“I-if you don’t I-I’ll kick you,” Error threatens weakly. The scare factor is considerably reduced, since he’s laying there (figuratively) boneless, his face a kaleidoscope of messy colors and his entire torso exposed. 

Blue just laughs softly. “Needy.” Considering Error’s (again, figuratively) boneless state, Blue slips one hand beneath the small of his back and lifts him slightly, using the other hand to tug down his shorts. Error grumbles something about not being helpless, but he doesn’t make any move to help Blue. 

With his shorts now around his ankles, and his entire body exposed, Blue thinks Error paints an awfully pretty picture. It strikes him that when they’d first begun dating this wouldn’t have been possible, and also that Error would never allow anyone else to have power over him this way. 

Blue wouldn’t abuse it. It was too precious a thing to have. 

He leans up close to Error again, nuzzling his cheek. Error still has his hands up by his face, although not necessarily covering it; his eyes watch Blue with interest, the multicolored lights in the sockets hazy with arousal. Blue grabs the hand still near his cheek, not quite holding it, simply laying his own hand on top of it, white bones on black. 

“I’m going to make you feel so good,” Blue promises, bringing his other hand down to Error’s pussy. Error twitches a bit as Blue cups his mound, letting his middle finger drag along the slit. His other hand properly cups Error’s cheek now, distractedly rubbing his thumb in circles, although they get clumsy as Error nuzzles into his hand. 

“So good,” Blue continues as he easily moves his fingers through the wetness coating Error’s conjured pussy. “Stars, you’re being so good…” 

“B-blue…” Error whines, pushing his hips up into Blue’s hand. Blue grins, and grinds his whole palm into Error’s clit, slowly, even while his fingers continue rubbing at Error’s folds. Error only whines some more at the dual stimulation, music to Blue’s ears. 

“I’ve got you, I’m going to take good care of you,” Blue promises. There’s a sound like a broken printer as he grinds his palm down again, and then Error’s arms are looped around his neck, pulling Blue down against his body. 

Blue continues by feel alone, his face buried against Error’s neck. Here, he can feel the vibrations of the faint purr that’s been coming from Error this whole time. It stutters when Blue moves his fingers up to focus on Error’s clit, his slick hand rubbing slowly across it a few times. 

Without seeing, it’s hard, but with Error clinging to him so tightly Blue is loath to move, so he settles for clumsily spreading Error’s folds with just two fingers. In his ear, Error whines, a high pitched noise that sounds less like a monster and a bit more like a computer, so Blue figures he’s getting close. 

He lets his teeth graze Error’s neck as he continues, messily using another two fingers to push back Error’s clitoral hood. It leaves Blue only one finger to rub at his clit, but that’s fine, because his clumsy circles are clearly doing wonders for Error, and he writhes under Blue between the attention on his pussy and the laving of Blue’s tongue on his neck. Between licks and nips, Blue keeps up a steady stream of mumbled praise, though he’s not sure it makes any sense. 

But soon Error’s whole body twitches, and his hips buck up once, twice into Blue’s hand. His mouth lets out a soft moan that turns into a mess of beeping, like a dial-up sound. Blue rubs him through it, much slower and a little gentler, and then stops as soon as he feels Error’s femurs stop twitching with his touch. 

He rolls to the side a bit, not completely away from Error, but far away enough to give him space if needed. It delights Blue to no end to see that Error hasn’t crashed - despite the odd computer-like sounds, Error is still conscious, panting like a dog. 

“You okay?” Blue asks softly. He doesn’t touch Error again, not yet - the other still hasn’t spoken. 

“...F-fuck,” Error replies eloquently, and Blue just smiles. He leans on his side and just watches as Error calms himself, still sitting there with his clothes haphazardly half-removed and his legs open. 

“How was it?” Blue pushes, trying to get a more verbal response than a single curse word. Error doesn’t move his body, but his eyes slowly drift to Blue. 

“G-good,” he mumbles. Blue watches him glance quickly downwards, where he’s sure there’s a cyan glow to match the bulge in his own pants, and then back up. “...I d-don’t think I h-have the energy for anything else today.” It’s as apologetic as Blue’s ever seen him, but it’s completely unneeded.

“That’s perfectly fine!” Blue assures him, nuzzling purposefully into his still-covered shoulder. Error’s body fizzles a bit, but that’s all, and Blue can’t stop himself from adding, “You did so well, and I’m really, really proud of you!” 

“...Stop it,” Error protests weakly. 

“I’m serious!” Blue replies, but he changes the subject and continues with his assurances, “I can just take care of myself later, don’t worry.” 

Error still looks unconvinced, but he knows as well as Blue does that he has no energy for anything else right now, even if Blue were still in control. 

“Or,” Blue purrs after a moment of thought, “you could just return the favor another day, if it means so much to you? I _have_ been wondering what you could do with those strings of yours…” 

Error says nothing, but Blue can tell, beneath the deep, colorful flush covering his face, that he is very, very seriously thinking about that.

**Author's Note:**

> tumblr @asriells  
> twitter @avosettas


End file.
